1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copier, a facsimile machine or a complex machine, and particularly to a construction of a fixing unit for fixing a toner image, which has been transferred to a sheet, to the sheet.
2. Description of the Related Art
A fixing unit of an image forming apparatus is adapted for fixing a toner image to a sheet by conveying the sheet having an unfixed toner image transferred thereto through a nip between a heating roller and a pressure roller while heating the unfixed toner image by means of the heating roller. In order to increase the temperature of the outer surface of the heating roller, an induction heating method for causing the heating roller to generate heat itself is adopted in addition to a radiant heating method according to which a heat source such a halogen lamp is installed in the heating roller. By this induction heating method, the temperature of the heating roller can be quickly increased by an eddy current resulting from electromagnetic induction.
As a prior art relating to a fixing unit adopting the induction heating method, a construction is known in which a bobbin having an induction heating function is installed in a cylindrical heating roller (see, for example, Japanese Unexamined Patent Publication No. 2002-311729). The bobbin also has a cylindrical shape and an induction coil, a ferrite core and the like are arranged therein. Since the heating roller and the bobbin are separate parts and wiring with the induction coil is arranged in the bobbin, the heating roller can freely rotate. Particularly in this prior art, one end of the bobbin is connected with one side wall plate of a main body of an image forming apparatus. Thus, upon detaching the fixing unit, the bobbin remains in the main body of the image forming apparatus. Therefore, the fixing unit is thought to be able to be easily disassembled and reassembled at the time of replacing the heating roller.
However, in the prior art construction in which the bobbin remains in the main body of the image forming apparatus, a circuit board and the like for supplying a current to the coil also remains in the main body together with the bobbin. Thus, for the maintenance, inspection and the like of the circuit board, an operator has to detach, replace and/or attach the circuit board while stretching his arms to the back of the main body of the image forming apparatus after the fixing unit is detached. Such an operation is particularly difficult if a space after the detachment of the fixing unit is small.